1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Cleanerless process dispensing with a cleaner such as cleaning blade or the like adjacent to the surface of a photoreceptor is advantageous in terms of the miniaturization of apparatuses and toner savings, and technologies for developing with simultaneous cleaning in reversal development have been heretofore disclosed. Such technologies for developing with simultaneous cleaning can be used effectively for the recent full-color image formation by an image forming apparatus and they are going to be employed in 4-unit tandem type image forming apparatuses.
A tandem type image forming apparatus is so constructed that a toner image transferred to a transfer belt or a sheet at an upstream image forming station is rushed as it is to its downstream image forming station. During this rushing, depending on the conditions, part of the toner transferred to the transfer belt or the sheet in the upstream image forming station may be reverse-transferred to the surface of a photoreceptor of the downstream image forming station. If no cleaner is available for cleaning the surface of the photoreceptor, the reverse-transferred toner is collected and recovered into the developing device of the downstream image forming station, with the result that color mixing occurs.
It has been known that the reverse transfer of toner tends to less occur with an increase in toner releasability of the photoreceptor of the downstream image forming station. U.S. Pat. No. 5,797,070 discloses measurement of the toner releasability of the photoreceptor by way of the contact angle of the surface of the photoreceptor for water, as well as an image forming apparatus having a photoreceptor with a surface having a contact angle for water of not less than 85°. Some other relevant prior art publications disclose toner particle diameter distribution based on the relationship between photoreceptor and toner (U.S. Pat. No. 5,731,122) and provision of a brush charger in combination with a photoreceptor (Japanese Patent Application KOKAI Publication No. H8-152786). Further prior art discloses an arrangement of yellow unit at an upstream image forming station so that it is made difficult to recognize color mixing even if such mixing occurs at a downstream image forming station (Japanese Patent Application KOKAI Publication No. H6-274002). As shown in Japanese Patent Application KOKAI Publications Nos. 2000-235311 and 2000-298388, a system for a tandem type image forming apparatus is proposed wherein a toner image on a photoreceptor is once transferred to an intermediate transfer member, whereupon the toner image is transferred to a sheet. According to this system, the intermediate transfer member has a cleaning member for cleaning the surface of the intermediate transfer member which is in direct contact with the sheet so that ingress of foreign matters such as paper powder into the region of photoreceptor is prevented.
In addition, in an image forming apparatus of cleanerless type which employs a combination of contact electrification by a magnetic brush using mainly magnetic carrier and two-component development using magnetic carrier and toner in the developing device, there is a problem in that the magnetic carrier is attached to the surface of the photoreceptor thereby to scratch the surface. This is due to rotation of the photoreceptor having on the surface thereof carrier which is harder and larger in size than toner and stuck to the surface of the photoreceptor having no cleaner, and this problem tends to occur in an apparatus having a transfer device of contact transfer type. To solve the problem, for example, Japanese Patent Application KOKAI Publication No. 2001-337548 discloses provision of a pressure (linear pressure) between the photoreceptor and the transfer member at 100 g/cm or less.
Although the various provisions have been thus proposed for the solution of the problems associated with toner color mixing and scratching of photoreceptor surface by carrier, the cleanerless process has other problems.
Since the photoreceptor has no cleaner for cleaning its surface in the cleanerless process, abrasion by a cleaner blade dose not occur on the photoreceptor surface even after a long-time use of the photoreceptor and, therefore, the cleanerless process contributes to extended serviceable life of the photoreceptor. On the other hand, however, any hard dirt stuck on the photoreceptor surface cannot be removed. Repeated image forming operation may allow unwanted residual toner on the photoreceptor to adhere its surface, thereby causing a trouble in image forming operation. This trouble is referred to as filming of photoreceptor.
The filming of photoreceptor tends to occur when a process such as contact electrification, contact transfer and contact development in which contact with a photoreceptor takes place is used. Of these, electrification and development in a non-contact or soft contact manner can be performed comparatively easily.
However, the transfer operation is accomplished on the precondition that there is contact between the photoreceptor and the medium to which an image is to be transferred, and, particularly in a four-unit tandem apparatus, the transfer is accomplished through contact of a transfer belt, a paper sheet conveyed by a transfer belt or an intermediate transfer member with the photoreceptor. If there is even a slight difference in surface speed between the photoreceptor and the transfer member or if the pressure to be applied during the transferring is too strong, part of toner and of additive substances to the toner are attached gradually to the surface of the photoreceptor and finally stuck hard on the surface to such an extent that they are difficult to be removed.
Formation of the filming on photoreceptor varies depending on toner composition, as well as the above conditions. The filming tends to occur, for example, when color toners with addition of more external additives are used for reproduction of glossy image or extended color reproduction.
As measures for inhibiting the filming, the above-cited intermediate transfer method may be used wherein an intermediate transfer member having a cleaner for removing paper power is provided so as to prevent paper powder from being attached to the photoreceptor. Although this method can improve the situation to a certain extent, the improvement cannot be said to be enough.
Further measures for preventing the filming on photoreceptor in an image forming apparatus other than the cleanerless type is disclosed by Japanese Patent Application KOKAI Publication No. 2000-155501, according to which the transfer belt is moved in contact with the photoreceptor with a difference in surface speed during the time when image forming operation is not performed, so that dirt is rubbed off from the photoreceptor surface. In such structure, however, the surfaces of the photoreceptor and the transfer belt are susceptible to scratches, which hampers the extended serviceable life of the photoreceptor that is one of the advantages of the cleanerless process.